Composite metal materials are obtained by mixing single-walled carbon nanotubes, multi-walled carbon nanotubes, nanocarbon fiber, fullerenes, or other nano-sized carbon materials (referred to below as “nanocarbon materials”) into metal alloys. Composite metal materials are thought to be capable of having enhanced mechanical and thermal properties relative to simple metal alloys.
However, nanocarbon materials have poor wettability in relation to metal alloys. The two materials will therefore separate if a nanocarbon material is simply stirred together with a metal alloy. Once separation has occurred, a composite metal material having the desired mechanical and thermal properties will not be able to be obtained. Techniques for preventing separation have already been proposed in, e.g., Japanese Patent Laid-Open Publication No. 2004-136363 (JP-A-2004-136363).
In claim 1 of JP-A-2004-136363, there is defined the invention “a method for molding a composite of a nanocarbon material and a metal alloy having a low melting point, comprising: cooling the melted metal alloy having a low melting point so that a liquid phase and a solid phase coexist and a thixotropic half-melted state is obtained; mixing the metal alloy having a low melting point and the nanocarbon material in this state and making a composite material; maintaining the thixotropy of the composite material and injecting the composite 1 material to fill a mold using a molding machine provided with heating means; and molding a composite metal article using the mold.”
In other words, a nanocarbon material is mixed into a metal alloy in a state in which both liquid and solid phases are present, and movement of the nanocarbon material is therefore limited. Since movement is limited, the nanocarbon material will not float up or precipitate out, and improvements in dispersibility can be achieved.
However, the metal alloy does not adhere to the nanocarbon material. Gaps may arise between the metal alloy and the nanocarbon material when repeated loads are applied to the composite metal material. When gaps arise, the mechanical and thermal properties deteriorate.
Further improvements in wettability have been needed as a counter measure, because the metal alloy can be made to adhere to the nanocarbon material if the wettability is good.
Techniques for further improving wettability have been proposed in, e.g., Japanese Patent Laid-Open Publication No. 2004-176244 (JP-A-2004-176244).
JP-A-2004-176244 is characterized in that a nanocarbon material to be added to a metal matrix is graphitized.
In order to verify the technique of JP-A-2004-176244, the present inventors performed an experiment for obtaining composite-metal molded articles in which a graphitized nanocarbon material was mixed into a metal alloy. The conditions and results of the experiment are as below.
Materials:
Metal alloy: ASTM AZ91D (magnesium alloy die-cast, equivalent to JISH 5303 MDC1D). The composition of a material specified as AZ91D is approximately 9% by mass of Al, 1% by mass of Zn, with the remainder being Mg and small amounts of other elements and unavoidable impurities.
Nanocarbon material: Graphitized nanocarbon material.
Mixing ratio: Shown in the following table.
Stirring: Three to five hours with a stirrer.
Injection Molding:
Size of the mold cavity:JIS 5 piece(65-mm length×27-mm width×3-mm thickness)
Injector type: Metal molding machine
Injection pressure: 20 MPa
Melting temperature: 590 to 600° C.
Injection rate: 1.5 m/s
Tensile Testing Machine:
Testing machine made by Shimadzu Corporation (AUTOGRAPH AG-250KNIS)
The tensile yield strengths (the value defined by JIS K7113 as “the tensile stress at the first point on a load/elongation curve at which an increase in length is recognized without an increase in load”) obtained using the tensile testing machine are shown in Table 1 below.
TABLE 1Composite materialGraphitizednanocarbonTensile yieldSample No.AZ91DmaterialstrengthSample 1 100%  0%190 MPaSample 299.9%0.1%190.2 MPa  Sample 399.5%0.5%191 MPaSample 499.0%1.0%192 MPaSample 598.5%1.5%206 MPaSample 698.3%1.7%198 MPaSample 798.0%2.0%192 MPa
The test piece in Sample 1 was manufactured using only AZ91D (magnesium alloy). The tensile yield strength was 190 MPa.
The test piece in Sample 2 was manufactured by mixing 0.1% by mass of nanocarbon material into 99.9% by mass of AZ91D (magnesium alloy). The tensile yield strength was 190.2 MPa.
The test pieces in Samples 3 and 4 were manufactured by mixing 0.5% and 1.0% by mass of nanocarbon material into 99.5% and 99.0% by mass of AZ91D (magnesium alloy). The tensile yield strengths were 191 MPa and 192 MPa.
The test piece in Sample 5 was manufactured by mixing 1.5% by mass of nanocarbon material into 98.5% by mass of AZ91D (magnesium alloy). The tensile yield strength was 206 MPa.
The test pieces in Samples 6 and 7 were manufactured by mixing 1.7% and 2.0% by mass of nanocarbon material into 98.3% and 98.0% by mass of AZ91D (magnesium alloy). The tensile yield strengths were 198 MPa and 192 MPa.
The tensile yield strength obtained using Sample 1 (190 MPa) will be used as a standard. Since the goal of adding a nanocarbon material and making a composite is to improve strength, an improvement in strength of at least 5%, and preferably 10% or more, is expected. 190 MPa (Sample 1) multiplied by a factor of 1.05 is 200 MPa, and 190 MPa (Sample 1) multiplied by a factor of 1.1 is 210 MPa.
The results were that Samples 2 through 4, 6, and 7 were less than 200 MPa. Sample 5 exceeded 200 MPa but was less than 210 MPa.
It should be noted that nanocarbon materials are extremely expensive.
The tensile yield strengths of Samples 2 through 7 are too low for the proportions of expensive nanocarbon material that were mixed. A technique that can yield a stronger molded article is needed to make effective use of expensive nanocarbon material.
The present inventors once again investigated graphitized nanocarbon materials, the use of which has become common knowledge. Specifically, nanocarbon materials are composed of regular six-membered rings (annular structures composed of six carbon atoms) or five-membered rings (annular structures composed of five carbon atoms). Nanocarbon materials having few defects can be obtained by graphitization. However, wettability is poor when graphitized materials having few defects are combined with a metal. The graphitized nanocarbon material may be further processed in order to resolve this drawback, but manufacturing costs will increase in proportion to the number of additional steps.
The present inventors therefore devoted themselves to developing a manufacturing method for providing a high-strength metal-composite molded article without raising manufacturing costs.
The inventors first observed the surface of a graphitized nanocarbon material using scanning electron microscopy (SEM). This revealed that the surface of the graphitized nanocarbon material is smooth. A further analysis using an X-ray diffraction apparatus revealed that the graphitized nanocarbon material has high crystallinity. Since it is smooth and has high crystallinity, the graphitized nanocarbon material is assumed to have low wettability with metal alloys. It is presumed that bonding between the metal alloy and the nanocarbon material will be incomplete if wettability is low, and improving strength will be difficult.
In order to improve wettability, the inventors observed a nongraphitized nanocarbon material using scanning electron microscopy while investigating various techniques for processing the surface of the nanocarbon material. The surface of the nongraphitized nanocarbon material was recognized as being rough. A further analysis using an X-ray diffraction apparatus revealed that the nanocarbon material is amorphous.
The strength of the nongraphitized nanocarbon material is low. Hence, such nongraphtized nanocarbon materials had not been considered as reinforcing materials. However, since they are amorphous and their surfaces are rough, such nongraphitized nanocarbon materials are assumed to have high wettability and their bondability with a metal alloy is expected to be adequate.
Adequately high strength was obtained when a nongraphitized nanocarbon material was stirred together with a metal alloy in accordance with the perspective above. This has lead to the present invention as summarized below.